How PDCP in LTE?
The main services provided by the Packet Data Convergence Protocol (PDCP) are:
For the user plane: 
  • IP header compression and decompression with the Robust Header Compression (ROHC) method only.
  • Ciphering
  • Transfer of user data
  • In-sequence delivery of upper layer PDUs at HO in the uplink
For the control plane: 
  • Ciphering and Integrity Protection to secure the transmission of core network signaling
The main services and functions of the PDCP sublayer for the user plane include:
  • Header compression and decompression: ROHC only;
  • Transfer of user data;
  • In-sequence delivery of upper layer PDUs at PDCP re-establishment procedure for RLC AM;
  • Duplicate detection of lower layer SDUs at PDCP re-establishment procedure for RLC AM;
  • Retransmission of PDCP SDUs at handover for RLC AM;
  • Ciphering and deciphering;

The header of an IP packet is 20 octets.

  • For example, during an FTP transfer, a lot of IP packets are sent over the air interface and the IP headers are almost always the same. They represent a significant amount of data which can be reduced thanks to a compression method.
  • The PDCP header is 1 (or 2) octet.
  • The Robust Header Compression (ROHC) is a standardized method used to compress IP, UDP, TCP and RTP headers.
  • RFC 4995